1. Field of the Invention
The present invention relates to a charged particle beam irradiation system and a charged particle beam extraction method. More particularly, the present invention relates to a charged particle beam irradiation system and a charged particle beam extraction method suitable for use as a particle beam therapy system in which an ion beam of, e.g., protons and carbon ions, is irradiated to an affected part in the body of a patient for treatment.
2. Description of the Related Art
There is known a therapy method for irradiating an ion beam (charged particle beam) of, e.g., protons or carbon ions, to an affected part of in the body of a patient, such as a cancer. An ion beam irradiation system for use in such therapy comprises a circular accelerator, a beam transport, and rotating irradiation equipment which includes an irradiation apparatus. The circular accelerator accelerates an ion beam circulating along an orbit to a target level of energy. The ion beam having been accelerated to the target level of energy is transported to the irradiation apparatus through the beam transport. The irradiation apparatus irradiates the ion beam after it has been formed in match with the shape of the affected part of the patient body. Known examples of the circular accelerator include means for circulating the ion beam along an orbit, means for bringing betatron oscillation of the ion beam into a resonant state outside the separatrix of resonance, and a synchrotron provided with a beam-extraction deflector for taking the ion beam out of the orbit.
The principle of the therapy using an ion beam is based on not only a characteristic that, immediately before the time when ions are stopped, most of energy of the ion beam is released and a dose distribution called a Bragg curve is formed, but also a characteristic that the position of the peak of the Bragg curve, i.e., the Bragg peak, in the direction of depth toward the inner side of the patient body from the body surface can be controlled depending on the magnitude of energy of the ion beam entering the patient body. By utilizing those characteristics, the energy of the ion beam is properly selected so that the ion beam is stopped near the affected part of the patient body and most of the ion beam energy is given to cancer cells of the affected part.
The Bragg peak has a width of several millimeters in the direction of depth (i.e., the direction of beam advance). Usually, the affected part has a thickness larger than the width of the Bragg peak in the direction of depth. In order to effectively irradiate the ion beam over the entire thickness of the affected part in the direction of depth, the energy of the ion beam (beam energy) and the dose of the ion beam must be controlled so as to form a wide and flat dose distribution corresponding to the size of the affected part.
From that point of view, in a known ion beam irradiation system, a range modulation wheel (hereinafter abbreviated to an “RMW”) is installed in an irradiation apparatus (see, e.g., Non-Patent Document 1; “REVIEW OF SCIENTIFIC INSTRUMENTS” (August 1993; FIG. 30). The RMW has a structure having a plurality of energy absorbers (blades) in the form of wedges with an axial thickness changing step by step in the circumferential direction. The RMW is arranged to cross a beam path within the irradiation apparatus and is rotated in a plane perpendicular to the beam path. During the rotation of the RMW, when the ion beam passes through a thinner portion of the blade, the beam energy is less attenuated and the Bragg peak is produced at a deeper position inside the body. When the ion beam passes through a thicker portion of the blade, the beam energy is attenuated to a larger extent and the Bragg peak is produced at a shallower position nearer to the body surface. Further, the position of the Bragg peak varies cyclically with the rotation of the RMW. As a result, a wide and flat dose distribution (called Spread Out Bragg Peak: SOBP) can be obtained over a region from the shallow position near the body surface to the deep position inside the body, looking at the beam energy integrated over time.